Refactor "temporary" Layers

[mbeckem]

Various packages currently need (mostly temporary) OpenLayers layers to implement their functionality.
Examples are measurement (for measurement geometries), map model (for highlight geometries) and geolocation (for the location geometry).

These layers share some common properties:

• They are considered internal (e.g. should usually not be shown in the TOC)
• They are "always on top"
• They are vector layers

The currently existing implementations create simple ad hoc OpenLayers layer instances (i.e. new VectorLayer()) and assign a very high z-index for the "always on top" feature. In other words, this approach currently bypasses our map model.

Instead of using raw OpenLayers instances, we should use "real" layer objects instead:

• This is an opportunity to handle "always on top" centrally and remove the z-index hack
• It enables future features such as showing a sketch layer in the TOC under certain circumstances